# Optical Activity/Polarimetry

## Main Question or Discussion Point

Hi,

I am a 17 year old college student from the UK carrying out an experiment to build a polarimeter to determine the specific rotation of sugars. However, I have a problem when I shine polarised light through my sucrose solution and turn the analyser polariser filter I see colours ranging from clear to yellow to blue and back again - rather than no light which I would expect when two polarisers are crossed. I have been researching for weeks to discover the root of this problem but I cannot find any mention of different colours! If possible, could you explain the theory as to why this works? I understand that chiral molecules rotate the plane of polarised light but I do not understand why I see different colours? As I haven't taken a physics A-level I find this very complicated. So far I have come across a number of things that may be to do with it (relevant or not). These are: ‘circular birefringence’, ‘circular dichroism’, ‘optical rotatory dispersion’, ‘left/right circularly polarised light’, ‘anisotropy’ and ‘isotropy’. I would appreciate a full explanation that isn't very complicated like some of the resources I find!. Thank you very much!

Tom

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Ibix
You know about refraction, where a ray of light bends when it enters a material with a different refractive index.

Some "optically active" crystals have a refractive index that depends on the angle between the polarisation direction of incident light and the symmetry axes of the crystal. What that turns out to mean in practice is that a polarised light beam striking such a crystal may come out with a different polarisation direction.

How does that affect your polarimeter? The first polar produces light polarised in the x-direction, for the sake of argument. The light passes through the crystal and rotates the polarisation direction (say) 15° clockwise. You can rotate the second polar to find the angle at which the light completely disappears - 15° clockwise of the y axis, in this case. As you rotate the polar, the amount of light you see varies from nothing to full brightness and back down to nothing every 180°.

So where do the colours come in? Basically, refractive index depends on wavelength - very strongly in some materials, less so in others. This means that the angle that the crystal rotates the light depends on the wavelength. So red light might rotate 15° and blue light 20°. Then, as you rotate the second polar, the amount of red light and the amount of blue light vary at different angles. So at some angles you see red light; at some angles, blue; at others, some red and some blue (purple).

Does that help to answer you?

Thank you giving a detailed explanation and a very quickly too. I just have a few more questions if that's okay. When taking a polarimeter measurement do you ignore the colours and take a minimum light reading anyway? for some reason not all the light is lost when I have rotated the second polar. Also when you said differwent wavelengths of light get rotated by different amounts; if you use a monochromatic light source of a very narrow wavelength such a sodium lamp/LED light will the wavelength not be refracted by different amounts therefore not showing different colours? Is there any way of calculating the colours that are shown using refrative indices? Sorry if these questions appear stupid...

Thanks,
Tom